<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <meta http-equiv="X-UA-Compatible" content="ie=edge" />
    <title>Document</title>
    <script src="lib/vue-2.4.0.js"></script>
  </head>
  <body>
    <div id="app">
      <input type="text" v-model="n1" />

      <select v-model="opt">
        <option value="+">+</option>
        <option value="-">-</option>
        <option value="*">*</option>
        <option value="/">/</option>
      </select>

      <input type="text" v-model="n2" />

      <input type="button" value="=" @click="calc" />

      <input type="text" v-model="result" />
    </div>
    <script>
      var vm = new Vue({
        el: "#app",
        data: {
          n1: 0,
          n2: 0,
          result: 0,
          opt: "+"
        },
        methods: {
          calc() {
            switch (this.opt) {
              case "+":
                this.result = parseInt(this.n1) + parseInt(this.n2);
                break;
              case "-":
                this.result = parseInt(this.n1) - parseInt(this.n2);
                break;
              case "*":
                this.result = parseInt(this.n1) * parseInt(this.n2);
                break;
              case "/":
                this.result = parseInt(this.n1) / parseInt(this.n2);
                break;
            }
          }
        }
      });
    </script>
  </body>
</html>
